High transmittance single crystal YAP scintillators

ABSTRACT

A single crystal yttrium aluminum perovskite scintillator has a minimum thickness of at least 5 mm and a transmittance of at least 50% at a wavelength of 370 nm. A method for fabricating the yttrium aluminum perovskite scintillator includes acquiring a yttrium aluminum perovskite single crystal boule, annealing the yttrium aluminum perovskite single crystal boule in an oxygen containing environment to obtain a partially annealed crystal, and annealing the partially annealed crystal in an inert environment or a reducing environment to obtain the yttrium aluminum perovskite single crystal scintillator.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of pending U.S. patent applicationSer. No. 15/965,079, filed Apr. 27, 2018, now U.S. Patent PublicationNo. 2018/0246231, published Aug. 30, 2018, which is a continuation ofU.S. Pat. No. 9,958,555, issued May 1, 2018, which is a divisional ofU.S. Pat. No. 9,541,669, issued Jan. 10, 2017, which are herebyincorporated herein in their entireties by reference.

FIELD OF THE INVENTION

Disclosed embodiments relate generally to single crystal scintillatorsand more particularly to yttrium aluminum perovskite (YAP) singlecrystal scintillators and methods for fabricating the scintillator.

BACKGROUND INFORMATION

Scintillators are used widely in the detection and spectroscopy ofenergetic photons (e.g., x-rays and gamma rays). They are commonly usedin nuclear and high energy physics research, medical imaging, anddownhole logging operations (e.g., operations in which gamma raymeasurements are used to determine properties of subterraneanformations).

Single crystal Yttrium Aluminum Perovskite (e.g., YAlO₃:Ce³⁺) is onepromising scintillating material. However, such crystals are known to becharacterized as having both a spatially non-uniform performance and alow transmittance due to the presence of color centers in the crystal.The color centers (F-centers and F⁺-centers) are believed to causeself-absorption of light emitted by the Ce³⁺ luminescence centers. Thecolor centers may be concentrated in certain regions of the crystal(e.g., in the lower and internal portions of the crystal, for example asdepicted on FIG. 1), depending on crystal growth conditions. Since lightoutput efficiency is an important scintillator property, the presence ofthe color centers tends to limit the utility of Yttrium AluminumPerovskite scintillators. Moreover, the presence of the color centerslimits the size of the crystal scintillators that may be utilized. In anattempt to overcome this difficulty very thin crystals have beenutilized in the prior art. For example, Zeng et al (Effects of Annealingon the Color, Absorption Spectra, and Light Yield of Ce:YAlO ₃ SingleCrystal Grown by the Temperature Gradient Technique, Journal of AppliedPhysics 95(2), 749, 2004) utilized crystals having a thickness of 1 mmand Cao et al (Effects of Growth Atmosphere and Annealing onLuminescence Efficiency of YAP:Ce Crystal, Journal of Alloys andCompounds 489, 515, 2010) utilized crystals having thicknesses of 0.36and 2 mm. At thicknesses greater than about 5 mm annealing has not beenpreviously shown to effectively remove color centers and other defectsfrom the crystals.

Therefore a need remains in the art for improved Yttrium AluminumPerovskite scintillators and methods for fabricating such scintillators.

SUMMARY

A single crystal yttrium aluminum perovskite scintillator having aminimum thickness of at least 5 mm and a transmittance of at least 50%at a wavelength of 370 nm is disclosed. A method for fabricating theyttrium aluminum perovskite scintillator is also disclosed. The methodincludes acquiring a yttrium aluminum perovskite single crystal boule,annealing the yttrium aluminum perovskite single crystal boule in anoxygen containing environment to obtain a partially annealed crystal,and annealing the partially annealed crystal in an inert environment ora reducing environment to obtain the yttrium aluminum perovskite singlecrystal scintillator.

The disclosed embodiments may provide various technical advantages. Forexample, the disclosed embodiments provide a bulk single crystal yttriumaluminum perovskite scintillator having a high transmittance and amethod for fabricating such as scintillator. The disclosed embodimentmay therefore significantly increase the efficiency of yttrium aluminumperovskite scintillators thereby enabling such scintillators to be usedin a broader range of applications, for example, including medicalimaging and downhole logging applications.

This summary is provided to introduce a selection of concepts that arefurther described below in the detailed description. This summary is notintended to identify key or essential features of the claimed subjectmatter, nor is it intended to be used as an aid in limiting the scope ofthe claimed subject matter.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the disclosed subject matter, andadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 depicts a prior art Yttrium Aluminum Perovskite (YAP) crystal.

FIG. 2 depicts a flow chart of one example of a method for fabricating aYttrium Aluminum Perovskite (YAP) crystal scintillator.

FIG. 3 depicts a flow chart of a more detailed embodiment of the methoddepicted on FIG. 2 for fabricating a Yttrium Aluminum Perovskite (YAP)crystal scintillator.

FIG. 4 schematically depicts one example of a suitable crystal growthmethod using the Czochralski method.

FIGS. 5A and 5B depict cuboidal (FIG. 5A) and cylindrical (FIG. 5B) YAPscintillator crystals obtained using the method depicted on FIG. 3.

FIG. 6 depicts a downhole nuclear logging tool deployed in asubterranean wellbore.

FIG. 7 depicts a plot of optical transmittance versus wavelength for oneexample of an as grown crystal, a partially annealed crystal, and afully annealed crystal.

FIG. 8 depicts a plot of optical transmittance versus wavelength foranother example of a fully annealed crystal.

DETAILED DESCRIPTION

FIG. 1 depicts a prior art Yttrium Aluminum Perovskite (YAP) singlecrystal 20. Such prior art crystals are known to include color centers30 distributed throughout the crystal. The color centers are oftennon-uniformly distributed through the crystal with lower portions of thecrystal including a higher concentration of color centers (the nature ofthe distribution is generally influenced by the crystal growthconditions). It will be understood that FIG. 1 is not intended to showthat there are no color centers in the upper portion of the crystal, butrather that the color centers tend to be non-uniformly distributed witha higher concentration in the lower portion of the crystal. The colorcenters are believed to cause self-absorption of light emitted by theCe³⁺ luminescence centers and therefore lower the transmittance of thecrystal. Since light output efficiency is an important scintillatorproperty, the presence of the color centers tends to limit the utilityof Yttrium Aluminum Perovskite scintillators. Moreover, as describedabove, the presence of the color centers limits the size of the crystalscintillators that may be utilized.

FIG. 2 depicts a flow chart of one example of a method 100 forfabricating a Yttrium Aluminum Perovskite (YAP) crystal scintillator. Inthe depicted embodiment, a yttrium aluminum perovskite single crystalboule is acquired at 110. The acquired crystal may then heat treatedusing a first annealing process in an oxygen containing environment at120 to obtain a partially annealed crystal. The partially annealedcrystal may then further heat treated in high vacuum or in a reducing orinert environment at 130 to obtain a fully annealed crystalscintillator.

FIG. 3 depicts a flow chart of an optional, more detailed embodiment 150of the method depicted on FIG. 2 for fabricating a YAP crystalscintillator. At 112 the yttrium and aluminum raw materials (e.g., theY₂O₃ and Al₂O₃ oxides) may be sintered, for example to burn off (remove)residual impurities prior to melting at 114 to form the melt. The YAPcrystal may be drawn or pulled from the melt at 116 as described in moredetail below. The as grown crystal may then be cut or cleaved orotherwise shaped to obtain a crystal preform (a single crystal of thedesired size and shape) at 118. The obtained crystal may then be heattreated using a first annealing process in an oxygen containingenvironment at 120 to obtain a partially annealed crystal. The firstannealing process may include spectroscopic measurement control 122 toprovide feedback regarding the length of the annealing step. Thepartially annealed crystal may then be further heat treated in highvacuum or in a reducing or inert environment at 130 to obtain a fullyannealed crystal scintillator. The fully annealed crystal may beevaluated at 140, for example, via measuring a transmission spectrum asdescribed in more detail below.

With continued reference to FIGS. 2 and 3, substantially any suitablecrystal growth process may be utilized. For example, the crystal growthprocess may include the Czochralski method in which the crystal is drawnor pulled from the liquid melt. FIG. 4 schematically depicts one exampleof a suitable crystal growth process. In the Czochralski method a seedcrystal 205 is introduced into the melt 210 on the end of a slowlyrotating metal rod 215 thereby initiating crystal growth (e.g., as shownat 220). Crystal growth continues as the rod is drawn upwards out of themelt (in a process called crystal pulling) as shown at 225. This processis commonly used to grow large silicon crystals for use in thesemiconductor industry. The YAP crystal grown by this process preferablyhas a <100>, <010>, or a <001> crystal orientation.

Other suitable crystal growth methods that may be employed at 110 mayinclude, for example, the Bridgman method, the vertical gradient freeze(VGF) method, the horizontal gradient freeze (HGF) method, and thegradient solidification method (GSM). In the Bridgman method an ampouleis used in which the melt and the crystal are contained. The ampoulecontaining the initially solid growth material is vertically movedupwards in the furnace so that the material melts top-down. At thebottom of the ampoule a mono-crystalline seed crystal is contained.After this seed crystal has been partly melted, the ampoule is slowlypulled back so that the crystal grows from the bottom up starting at theseed crystal.

In the VGF method several concentric heating circuits which aresuperimposed upon each other are arranged around the stationary meltingcrucible in jacket form. Each of these heating circuits can beseparately energized. By slowly decreasing the heat output of eachsingle heating circuit arranged around the crucible wall the temperaturecan be slowly decreased below the crystallisation point, thus producinga radial temperature gradient along which the crystal growth takesplace. The HGF method is similar to the VHF method with the exceptionthat the construction is rotated by 90 degrees. The GSM is also similarto the HGF and VHF methods with the exception that the heating circuitsare slowly moved during the crystallization process.

With reference again to FIGS. 2 and 3, the obtained crystal (e.g.,obtained using the Czochralski method depicted on FIG. 4) is heattreated using a first annealing step to obtain a partially annealedcrystal. In the first annealing step the crystal is annealed in anoxygen containing environment, for example, in air or other oxygencontaining gases at substantially any suitable oxygen concentration. Thetime length of the first annealing step generally depends on the size ofthe crystal and the annealing temperature with the time generallyincreasing with increasing crystal size and decreasing temperature. Thetime length may be, for example, at least about 30 minutes (e.g., atleast about 45 minutes or at least about 60 minutes). The time lengthmay be, for example, about 6 hours or less (e.g., about 4 hours or lessor about 3 hours or less). The time length may, for example, be in arange from about 30 minutes to about 6 hours (e.g., from about 45minutes to about 4 hours or from about 1 hour to about 3 hours).

The annealing temperature of the first annealing step may be, forexample at least about 1000 degrees C. (e.g., at least about 1050degrees C. or at least about 1100 degrees C.). The annealing temperatureof the first annealing step may be, for example, about 1500 degrees C.or less (e.g., about 1400 degrees C. or less or about 1300 degrees C. orless). The annealing temperature of the first annealing step may be in arange from about 1000 degrees C. to about 1500 degrees C. (e.g., fromabout 1050 degrees C. to about 1400 degrees C. or from about 1100degrees C. to about 1300 degrees C.).

The first annealing step may further include a multi-step process, forexample, including a two-step or a three-step process. For example, in atwo-step process the crystal may be placed in a furnace at a firsttemperature (e.g., ambient temperature) and heated with the temperatureof the furnace increasing over a period time to the annealingtemperature. The crystal may remain in the furnace at the annealingtemperature for a predetermined time before being removed. In analternative two-step process the crystal may be placed in a furnacealready pre-heated to the annealing temperature. After somepredetermined time the furnace may then be cooled to a secondtemperature (e.g., ambient temperature) over a period of time. In athree-step process, the crystal may be placed in a furnace at a firsttemperature (e.g., ambient temperature) and heated over a first periodtime to the annealing temperature. After some predetermined time at theannealing temperature the furnace may then be cooled to a secondtemperature (e.g., ambient temperature) over a second period of time. Insuch two-step and three-step processes, the first and second periods oftime may be of substantially any time length, for example in a rangefrom about 1 to about 8 hours.

With continued reference to FIGS. 2 and 3, the partially annealedcrystal is annealed in a second annealing step to obtain a fullyannealed crystal scintillator. The partially annealed crystal may beannealed in the second annealing step in substantially any inertenvironment, reducing environment, or mixture thereof. An inertenvironment may include any of the noble gases, nitrogen, and mixturesthereof. For example, an inert environment may include purified argon ornitrogen gas. An inert environment may also include an evacuatedenvironment, for example, at a pressure less than about 5×10⁻⁴ mbar. Areducing environment may include any reducing gases such as hydrogen,carbon monoxide, methane, ammonia, hydrogen sulphide, and mixturesthereof. For example, a reducing environment may include purifiedhydrogen gas.

The time length of the second annealing step also generally depends onthe size of the crystal and the annealing temperature with the timegenerally increasing with increasing crystal size and decreasingtemperature. The time length may be, for example, at least about 20hours (e.g., at least about 40 hours, at least about 80 hours, at leastabout 120 hours, or at least about 200 hours). The annealing temperatureof the second annealing step may be, for example, at least about 1500degrees C. (e.g., at least 1600 degrees C., or at least about 1700degrees C.). The annealing temperature of the second annealing step maybe, for example, about 2000 degrees C. or less (e.g., about 1900 degreesC. or less or about 1800 degrees C. or less). The annealing temperatureof the second annealing step may be in a range from about 1500 degreesC. to about 2000 degrees C. (e.g., from about 1600 degrees C. to about1900 degrees C. or from about 1700 degrees C. to about 1800 degrees C.).As with the first annealing step the second annealing step may include atwo-step or a three-step process, for example, as described above.

FIGS. 5A and 5B depict first and second example annealed YAPscintillator crystals obtained using the methods depicted on FIGS. 2and/or 3. In the example depicted on FIG. 5A, crystal scintillator 300has a cuboidal shape with dimensions a, b, and c. Embodiments of thecrystal fabrication methodology disclosed herein advantageously mayenable large dimension YAP crystal scintillators to be fabricated. Forexample, the crystal scintillator may have a thickness of at least 5 mm(e.g., at least 10 mm, at least 15 mm, or at least 20 mm). In thedepicted embodiment, crystal scintillator 300 may have a, b, and cdimensions of at least 5 mm (e.g., at least 10 mm, at least 15 mm, or atleast 20 mm). In the example depicted on FIG. 5B, crystal scintillator400 has a cylindrical shape in which both the length l and the diameterd of the crystal are at least 5 mm (e.g., at least 10 mm, at least 15mm, or at least 20 mm).

The disclosed YAP scintillator crystals may be further be characterizedas having a Ce³⁺ concentration of at least 0.02 weight percent and aCe⁴⁺ concentration of less than 0.1 weight percent (and preferably lessthan 0.01 weight percent). The annealed crystals further preferably havea <100>, <010>, or a <001> crystal orientation.

The annealed YAP scintillator crystals may be further characterized ashaving a high optical transmittance at wavelengths in a range from about340 to about 380 nm. Optical transmittance is a measure of the fractionof incident light at a specified wavelength or range of wavelengths thatpasses through a sample (i.e., that is not reflected or absorbed by thesample). The YAP scintillator crystals (grown and annealed as describedabove) have a transmittance of at least 50% at wavelengths in the rangefrom about 340 to about 380 nm. The transmittance may further be atleast 60%, at least 70%, or even at least 80% at wavelengths in therange from about 340 to about 380 nm.

The annealed YAP scintillator crystals may be still furthercharacterized as having a high photon yield (light yield) as comparedwith the prior art YAP scintillator crystals. The prior art crystals(having color centers) have a comparatively low photon yield due toself-absorption and intrinsic quenching. The annealed YAP scintillatorcrystals may have a photon yield up to twice that of the prior artcrystals. Moreover, the photon yield of the annealed YAP scintillatorcrystals has a high spatial uniformity (a low spatial non-uniformity) ascompared to the prior art YAP crystal scintillators. The non-uniformspatial distribution of the color centers in the prior art crystalsresults in a correspondingly non-uniform photon yield (i.e., the photonyield is much less at the lower end of the crystal than the upper end).In contrast to the prior art crystals, the disclosed YAP scintillatorcrystals have a photon yield with a high spatial uniformity (low spatialnon-uniformity). For example, the photon yield spatial non-uniformitymay be less than about 10 percent (e.g., less than about 5 percent, oreven less than about 2 percent). The photon yield spatial non-uniformitymay be obtained for example by comparing the photon yield at the top ofthe crystal with that photon yield at bottom of the crystal.

FIG. 6 depicts a downhole nuclear logging tool 500 deployed in asubterranean wellbore 501. Logging tool 500 includes a neutron source520 and at least one gamma ray detector 540 (e.g., near and fardetectors) deployed in logging tool body 510. The neutron source 520 mayinclude substantially any suitable source capable of emitting neutronsinto a surrounding formation 502 to produce inelastic gamma-rays. Forexample, the neutron source 520 may be a pulsed electronic neutronsource, such as a Minitron™ by Schlumberger Technology Corporation.Additionally or alternatively, the neutron source 18 may be aradioisotope source capable of emitting fast neutrons. The gamma raydetector(s) 540 include a YAP scintillator crystal. The YAP scintillatorcrystal may be fabricated, for example, using one of the methodsdescribed in FIGS. 2 and 3.

Nuclear logging tool 500 may be fabricated, for example, by deployingthe neutron source 520 and the gamma ray detector(s) 540 in a loggingtool body 510 such as a logging while drilling tool body or a wirelinelogging tool body. A YAP single crystal may be annealed as describedabove with respect to FIGS. 2 and 3 in an oxygen containing environmentto obtain a partially annealed crystal and then in an inert environmentor a reducing environment to obtain the YAP single crystal scintillatorwhich may be in turn deployed in a gamma ray detector and in the loggingtool body.

With continued reference to FIG. 6 logging tool 500 may further includean electronic controller (not shown). The controller may be configured,for example, to process gamma rays received by the gamma ray detector(s)540 to estimate at least one property of the formation 502. For example,the received gamma rays may be processed to obtain a thermal neutroncapture cross section (known in the art as “sigma”) and/or carbon oxygenratio of the formation. Such processing methodologies are disclosed inmore detail, for example, in U.S. Pat. Nos. 4,721,853 and 4,937,446,which are incorporated by reference herein in their entirety.

The electronic controller may be further configured to cause the neutrongenerator to emit neutrons into a subterranean formation (during alogging operation) and cause the gamma ray detector(s) to detect gammarays resulting from an interaction between the emitted neutrons and thesubterranean formation.

The disclosed embodiments are now described in further detail by way ofthe following example, which is not intended to be limiting in any way.

EXAMPLE 1

A single crystal YAlO₃:Ce³⁺ cylindrical boule having a length of 30 mmand a diameter of 40 mm was grown having a <100> orientation. The asgrown crystal had an absorption coefficient of greater than 2 cm⁻¹ at awavelength of 370 nm. The crystal was partially annealed in a firstannealing step as follows. The crystal was placed in an air containingfurnace at ambient temperature. The temperature was increased to 1200degrees C. over a period of six hours and then held at 1200 degrees C.for one hour. The furnace temperature was then cooled to ambienttemperature over a second period of six hours.

The crystal was then placed in a second furnace having tungsten heatingelements in a molybdenum crucible and covered with a sheet of molybdenumthermal shielding. The furnace was then evacuated to a pressure of 10⁻⁴mbar and then filled with argon gas to an absolute pressure of 1.2 bar.The crystal was then annealed in this atmosphere for 80 hours at 1700degrees C. Transmission spectra of the as-grown crystal, the partiallyannealed crystal, and the fully annealed crystal were obtained using aBrucker FTIR Spectrometer and AvaSpec-2048 Fiber Optic Spectrometer.These spectra are depicted on FIG. 7. The transmittance spectrum of theas grown crystal is depicted at 610. The transmittance spectra of thepartially and fully annealed crystals are depicted at 620 and 630. It isreadily apparent that the two-step annealing process described abovegreatly increases the transmittance of the YAP crystal over allwavelengths from about 350 to about 800 nm. For example, at 370 nm thetransmittance of the as grown crystal was about 5%. After the first airannealing step the transmittance was increased to about 35%. After thesecond annealing step has been completed as described herein thetransmittance was increased to nearly 80%.

EXAMPLE 2

A single crystal YAlO₃:Ce³⁺ cuboidal boule having dimensions of 20×20×25mm was prepared. The as prepared crystal had an absorption coefficientof greater than 2 cm⁻¹ at a wavelength of 370 nm. The crystal waspartially annealed in an air containing furnace at 1100 degrees C. fortwo hours. The crystal was then placed in a second furnace havingwolfram heating elements in a molybdenum crucible and covered with asheet of molybdenum thermal shielding. The furnace was evacuated to apressure of 10⁻⁴ mbar and then filled with hydrogen gas to an absolutepressure of 1.2 bar. The crystal was then annealed in this atmospherefor 200 hours at 1600 degrees C. A transmission spectrum of the fullyannealed crystal was obtained as described in Example 1 and is depictedon FIG. 8. As in Example 1 the two-step annealing process describedabove has greatly increases the transmittance of the YAP crystal overall wavelengths from about 350 to about 800 nm. For example, at 370 nmthe transmittance of the as fully annealed crystal was greater thanabout 60 percent.

Although high transmittance single crystal Yttrium Aluminum Perovskitescintillators and methods for fabricating such scintillators have beendescribed in detail, it should be understood that various changes,substitutions and alternations can be made herein without departing fromthe spirit and scope of the disclosure as defined by the appendedclaims.

What is claimed is:
 1. A method for making a scintillator comprising:forming a boule consisting essentially of single crystal yttriumaluminum perovskite (YAP) using the Czochralski process; and performinga first annealing of the single crystal YAP in an oxygen containingenvironment, and a second annealing of the single crystal YAP in aninert environment to make the scintillator.
 2. The method of claim 1,wherein the single crystal yttrium (YAP) has a thickness of at least 5mm, and a photon yield spatial non-uniformity less than 10 percent. 3.The method of claim 2, wherein the photon yield spatial non-uniformityis less than 5 percent.
 4. The method of claim 2, wherein the thicknessis at least 10 mm.
 5. The method of claim 1, wherein the single crystalYAP has a thickness of at least 5 mm, and a transmittance of at least50% at a wavelength in a range of 340 nm to 380 nm.
 6. The method ofclaim 1, wherein the first annealing is at a temperature in a range from1100 degrees C. to 1500 degrees C., and the second annealing is at atemperature above 1600 degrees C.
 7. A method for making a scintillatorcomprising: forming a boule consisting essentially of single crystalyttrium aluminum perovskite (YAP) using the Czochralski process; andperforming a first annealing of the single crystal YAP in an oxygencontaining environment, and a second annealing of the single crystal YAPin a reducing environment to make the scintillator.
 8. The method ofclaim 7, wherein the single crystal yttrium (YAP) has a thickness of atleast 5 mm, and a photon yield spatial non-uniformity less than 10percent.
 9. The method of claim 8, wherein the photon yield spatialnon-uniformity is less than 5 percent.
 10. The method of claim 8,wherein the thickness is at least 10 mm.
 11. The method of claim 7,wherein the single crystal YAP has a thickness of at least 5 mm, and atransmittance of at least 50% at a wavelength in a range of 340 nm to380 nm.
 12. The method of claim 7, wherein the first annealing is at atemperature in a range from 1100 degrees C. to 1500 degrees C., and thesecond annealing is at a temperature above 1600 degrees C.